This invention relates to an index information recording system for a picture information filing system using a magnetic tape device as picture information memory.
Picture information filing systems have recently been developed, in which picture information of a great deal of documents can be photoelectrically converted through two-dimensional optical scanning and then stored in a large capacity picture information memory so that it can be searched for and reproduced for recording or display as two-dimensional visible image when required. As the large capacity picture information memory which is used with such a picture information filing system for memorizing picture information, magnetic tape devices have recently been contemplated. An example of such device is disclosed in U.S. Pat. No. 3,946,959 which is applied by the same applicant.
In this magnetic tape device, an endless magnetic tape having a number of recording tracks extending in the direction of its running is moved at a high speed for recording picture information (video signal) obtained from picture information of a document or the like through two-dimensional optical scanning on the tape with a magnetic head for unit quantity (i.e., for one page) after another or searching for and reproducing recorded picture information. At this time, recording tracks are selected by moving the head in a direction perpendicular to the direction of running of the tape.
In such magnetic tape device, index information for each unit quantity of the picture information is also recorded together therewith, and this index information is used as searching code at the time of searching for desired picture information. Doing so makes it possible to obtain better operation control. To this end, it is thought to use at least one of the recording tracks as index recording track in which to record the index information of the picture information recorded in other tracks which serve as picture information recording tracks. However, in case where the index information is merely successively recorded in the index recording track, at the time when searching for the index information the running speed of the tape, which is being moved at a high constant speed, cannot be sufficiently followed, and sometimes it happens that some index information fails to be decoded. More particularly, since the running speed of the tape is very high compared to the decoding period involved in the searching for index information, failure of decoding of some items of index information is prone, that is, the running speed of the tape is so high compared to the decoding period for searching index information that it is likely that a portion of the tape corresponding to the next item of index information comes to the head during the decoding of an item of index information. Another deficiency occurs at the time when adding index information of newly recorded picture information. While the new index information is additionally recorded by detecting the last index information that has already been recorded, because of its very high running speed the tape is moved a considerable distance from the detection of the last index information till the commencement of recording of the new index information. Therefore, a considerable gap is produced between adjacent items of index information which are recorded one after another, that is, the individual items of index information are recorded in a discontinuous fashion.